1. Field of the Invention
The present invention relates to a semiconductor device having a plurality of semiconductor elements and a manufacturing method thereof.
2. Description of the Related Art
In a case where a plurality of semiconductor elements are provided over an insulating surface, a method is employed, by which a semiconductor film formed over an insulating surface is processed by etching processing into a plurality of island-shaped semiconductor layers. The semiconductor element has a stacked structure of a plurality of thin films, and in a case of a planar type thin film transistor, a gate insulating layer is stacked to cover semiconductor layers divided into island shapes.
The semiconductor layers processed into island shapes have steps in edge portions thereof, and accordingly, defects such as thinning of the gate insulating layer and breakage of a film are generated.
When the gate insulating layer is thinned, a characteristic defect of a semiconductor device is generated, such as leakage current flowing between a gate electrode and the semiconductor layer and a short-circuit (a short) caused by contact between the gate electrode and the semiconductor layer.
In order to solve the above-described problems, a method is conducted, in which two gate insulating layers having different shapes are stacked to relieve steps resulting from the edge portion of the semiconductor layer and to improve coverage (for example, see Patent Document 1: Japanese Published Patent Application No. H10-242471).